Method for defluorination of phosphoric acid



June 6, 1961 ZOO G. H. GLOSS METHOD FOR DEFLUORINATION OF PHOSPHORIC ACID Filed Dec. 27,

0.252 `522266Z 0.1% zZz'cQ United States Patent l 2,987,376 METHOD FOR DEFLUORINATION 0F PHOSPHORIC ACID Gunter H. Gloss, Mundelein, lll., assigner to International Minerals & Chemical Corporation, a corporation of New York Filed Dec. 27, 1956, Ser. No. 630,848 12 Claims. (Cl. 23-165) 'I'his invention relates to a new and novel method of removing impurities such as iluorine from acidic phosphate solutions. More particularly, it relates to the production of deuorinated wet process phosphoric acid.

Various grades of so-called superphosphates are produced in commerce. The fluorine content of these superphosphates preclude their use as an animal food supplement material. Phosphates of low uorine content have been prepared by solubilizing the phosphate Values of phosphate rock by reaction with sulfuric acid and treating the resultant aqueous solution with lime to remove impurities prior to precipitation of dicalcium phosphate.

This process has the disadvantage that appreciable quantities of P205 values are lost in the precipitate which removes uorine from solution, thereby diverting phosphate content from relatively high value animal feed product to relatively low valued fertilizer product.

Another method of deuorination heretofore in use has been the passage of super-heated steam through hot phosphoric acid either during the latter stages of concentration or through the concentrated acid. The gases evolved contained predominantly water and carbon dioxide plus small quantities of phosphoric acid, hydrouoric acid, and silicon tetrafluoride. Recovery of fluorine-bearing material from this mixture is a diicult problem. Hydrouoric acid tends to polymerize and is diiflcult to absorb in water even at low temperature. Consequently, recovery of uorine evolved from solutions is very low. Further, the vapor pressure of HF in an aqueous system is such that absorption at the normal operating temperature for scrubber equipment, i.e., 100 to 200 F., is highly inefficient.

It is a primary object of this invention to overcome the disadvantages and shortcomings of the processes heretofore in use.

It is an object of this invention to provide a process whereby phosphoric acid is more easily and more cheaply deiluorinated.

It is another object of this invention to provide a process wherein the fluorine is vaporized in a form rendering it recoverable in the form of iluosilicic acid.

It is still another object to provide a process wherein the uorine is obtained in a vapor form recoverable in absorbers or scrubbers operating at relatively high temperatures.

It is a further object to provide a process easily controllable for production of SiF4 vapors.

These and other objects will be apparent to those skilled in the art from the following description.

Now it has been discovered that concentrated wet process phosphoric acid of a concentration boiling at a temperature higher than 250 F. can be deuorinated by heating to boiling under conditions permitting little change in volume or acid concentration, in the presence of linely divided or colloidal silica, thereby causing water vapor and silicon tetrafluoride to be evolved, in which form the fluorine compounds are almost completely recoverable in water at temperatures up to about 200 F.

More in detail, phosphoric acid as manufactured by the reaction of phosphate rock with sulfuric acid contains impurities Such as A1203, Fe203, Ca0 and F. A typical chemical analysis of this acid is as follows:

Patented June 6, 1961 ICC Constituent: Wt. Percent P205 27.73 F 1.87 A1202 1.26 Fezog l. CaO 0.27 S04 1.3 Si02 0.95

Wet process phosphoric acid of this type is usually obtained as an acid of between about 26 and 30% P205 concentration. At this concentration, deiluorination by boiling is substantially negligible and the boiling technique herein described is not carried out until the acid has been concentrated to a point giving the acid a boiling point of about 250 F. The dilute phosphoric acid is generally concentrated to have `a boiling point in the range between about 250 F. and about 300 F. and preferably in the range between about 255 F. and 275 F.

To drive uorine from the acid solution, between about 0.3% and about 1.2% by weight of silica is added to the acid and the resulting slurry is heated to its boiling point. During the boiling operation, water is continuously added to the acid solution to maintain a substantially constant boiling mixture, i.e., an acid Whose P205 concentration will not iluctuate more than about 3% from the start of boiling until the nish of the heating operation, for example, between about 50 and about 60% corresponding, for this type of acid, to a mixture boiling at 255 F. to about 265 F. An effective boiling rate for evolution of SiF., requires a continuous introduction of water at a rate corresponding to between about l0 pounds of water per unit of P205 to about 30 pounds of water per unit of P205 per hour of boiling, a unit of' P205 meaning 20 pounds.

Fluorine evolution in the form of SiF4 varies almost directly with temperature. Comparable deuorination results are obtainable on acid of the above analysis by heating the silica-containing acid slurry for 30 minutes at 265 F. or for 90 minutes at 250 F.

When boiling acid for a iixed period of time at any particular temperature, the fluorine removal increases as the quantity of silica present is increased from about 0.1% to about 1% by Weight. The presence of silica in amounts exceeding 1% produces only slight increase in the amount of uorine vaporized from the solution. The effect of adding silica is illustrated by the curves in the igure in which the P/F weight ratio of the solution is plotted as the ordinate and the pounds of water added for unit of P205 is plotted as the abscissa. Phosphoric acid of 53% P205 was boiled at a temperature of 265 C. at 30 minute intervals in each instance of data taking. Silica was added in the form of a pulverized material having a surface area of the order of sq. meters per gram. In order to obtain a P/F weight ratio such that upon precipitation of the phosphate values in calcium phosphate form, the precipitate will have a uorine content suiciently low to be acceptable as an animal feed, the phosphate solution must have a P/F higher than about and preferably in the range of to 200. To obtain this degree of deuorination in a commercially feasible time such as 30 minutes, between about 0.5 and 1% of silica must be present in the boiling slurry.

To be effective in removing uorine from boiling phosphoric acid solutions, silica must be in a colloidal or semicolloidal form. Gelatinous silica such as may be precipitated from uosilicic acid solutions is an effective form of silica. Finely `divided forms of silica such as diatomaceous earth are effective for converting the iluorine content of the acid to silicon tetrauoride which will be vaporized from the solution along with water. In order to be eiective, inely divided silica must have a surface area in excess of about sq. meters per gram and preferably in the range between about 50 and 100 sq. meters per gram. Other useful forms of solid silica are micronized sand and powdered rock materials of a lrelatively high silica content such as perlite. The purity of the silica material has a marked effect upon the degree of deuorination; materials such as perlite are markedly more ecient in iluorine removal than the silicate clays such as bentonite. Y

The invention will be more fully understood from the following example.

Example Wet process phosphoric acid (400 parts by weight) having a P205 content of approximately 27.7% by weight, a iluorine content of 1.9%, and a P/l;` ratio of approximately 29, was concentrated'to 58% P205. To the concentrate were added 4 parts by weight of silica having 60 sq. meters of surface per gram of product, the added material being a material precipitated during the absorption of den gases in water. The resulting slurry was agitated and heated to boiling at 265 F. When boiling occurred, water addition was started and continued until the end of the boiling period of 60 minutes at a water rate of 12 pounds of water per unit of P205 in solution. At the end of 60 minutes, the acid concentration was approximately 58% P205. Analysis of the product acid was as Having thus described my invention, what I claim is:

l. The method of removing iiuorine from wet process phosphoric acid containing uorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point higher than about 250 F. to a boiling temperature in the presence of in excess of about 0.1% by weight of silica having a surface area in excess of 25 square meters per gram, boiling the resultant acid slurry whereby water vapor and silicon tetrafluoride are vaporized and removed while continuously adding water to maintain a substantially constant boiling composition boiling higher than about 250 F., withdrawing a phosphoric acid-silica slurry from the boiling operation having a P/F weight ratio in excess of 100, and removing silica from the deuorinated acid.

2. The method of claim 1 wherein the silica is a diatomaceous earth.

3. The method of removing fluorine from wet process phosphoric acid containing iiuorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point higher than about 250 F. to a boiling temperature, adding to the boiling acid silica having a surface area in excess of 25 square meters per gram in a proportion in excess of about 0.1% by weight of the resultant acid slurry, boiling the acid slurry, whereby water vapor and silicon tetrafluoride are vaporized and removed while continuously adding water to maintain a substantially constant boiling composition boiling higher than about 250 F., withdrawing a phosphoric acid-silica slurry from the boiling operation having a P/li` Weight ratio in excess of 100, and removing silica from the defluorinated acid.

4. The method of claim 3 wherein the added silica is a diatomaceous earth.

5. The method of removing uorine from wet process phosphoric acid containing fluorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point in the range between about 250 F. and about 300 F. to a boiling condition, adding to the boiling acid silica having a surface area in excess of 25 square meters per gram in a proportion in excess of about 0.1% by weight of the resultant acid slurry, boiling the acid slurry, whereby water vapor and silicon tetrafluoride are vaporized and removed while continuously adding Water to maintain a substantially constant boiling composition boiling higher than about 250 F., withdrawing a phosphoric acid-silica slurry from the boiling operation having a P/ F weight ratio in excess of 100, and removing silica from the deuorinated acid.

6. The method of removing fluorine from wet process phosphoric acid containing fluorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point in the range between about 255 F. and about 275 F. to a boiling condition, adding to the boiling acid silica having a surface area in excess of 25 square meters per gram in a proportion between about 0.1% and about 1% by weight of the resultant acid slurry, boiling the acid slurry, whereby water vapor and silicon tetrafluoride `are vaporized and removed while continuously adding water to maintain a substantially constant boiling composition boiling higher than about 250 F., withdrawing a phosphoric acid-silica slurry from the boiling operation having a P/F weight ratio in excess of 100, and removing silica from the defluorinated acid.

7. The method of removing fluorine from wet process phosphoric acid containing iluorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point higher than about 250 F. to a boiling temperature, adding to the boiling acid silica having a surface area in the range between about 50 and about square meters per gram in a proportion in excess of about 0.1% by weight ofthe resultant acid slurry, boiling the acid slurry, whereby water vapor and silicon tetrafluoride are vaporized and removed while continuously adding water to maintain a substantially constant boiling composition boiling higher than about 250 F., continuously removing said water vapor and silica tetrailuoride until the P/F weight ratio of said slurry is in excess of 100, withdrawing a phosphoric acid-silica slurry from the boiling operation having a P/ F weight ratio in excess of 100, and removing silica lfrom the defiuorinated acid.

8. The method of removing fluorine from Wet process phosphoric acid containing iiuorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point higher than about 250 F. to a boiling temperature, adding to the Vboiling acid silica having a surface area in excess of 25 square meters per gram and freshly precipitated from fluosilicic acid solution in a proportion in excess of about 0.1% by weight of the resultant acid slurry, boiling the acid slurry, whereby water vapor and silicon tetrailuoride are vaporized and removed while continuously adding water to maintain a substantially constant boiling composition, boiling higher than about 250 F., continuously removing said water vapor and silica tetrafluoride until the P/F weight ratio of said slurry is in excess of 100, withdrawing a phosphoric acid-silica slurry from the boiling operation having a P/F weight ratio in excess of 100, and removing silica from the deiluorinated acid.

9. The method of removing uorine from wet process phosphoric acid containing iluorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point higher lthan about 250 F. to a boiling temperature, adding to the boiling acid silica having a surface area in excess of 25 square meters per gram in a proportion in excess of about 0.1% by weight of the resultant acid slurry, boiling the acid slurry, whereby water vapor and silicon 4tetrafluoride are vaporized and removed while continuously adding water to maintain a substantially constant boiling composition boiling higher than about 250 F., continuously removing said water vapor and said silicon tetrafluoride until vthe P/F weight ratio of said slurry is in excess of 100, withdrawing a phosphoric acid-silica slurry lfrom the boiling operation having a P/F weight ratio in excess of 100,

and removing silica from the deuorinated acid, and returning the separated silica plus additional fresh silica to the boiling operation in quantities to maintain up to 1% by Weight in the resultant acid slurry.

10. The method of removing iluorine from wet process phosphoric acid containing uorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point higher than about 250 F. to a boiling temperature, adding to the boiling acid silica having a surface area in excess of 25 square meters per gram in a proportion in excess of about 0.1% by Weight of the resultant acid slurry, whereby water vapor and silicon tetrauoride are vaporized and removed while continuously adding water at a rate between and 30 pounds per unit of P205 per hour of boiling, continuously removing said water vapor and said silicon tetrafluoride until the P/F weight ratio of said slurry is in excess of 100, withdrawing slurry from Ilthe boiling operation having a P/P` weight ratio in excess of 100, and removing silica from the deuorinated acid, and returning the separated silica plus additional fresh silica to the boiling operation in quantities to maintain up to 1% by Weight in the resultant acid slurry.

11. The method of removing uorine from wet process phosphoric acid containing fluorine in undesirable amounts which comprises heating said acid at a concentration having a boiling point higher than about 250 F. to the boiling temperature, adding to the boiling acid silica having a surface area in excess of about 25 square meters per gram in a proportion in excess of about 0.1% by Weight of the resultant acid slurry, boiling the acid slurry, whereby water vapor and silicon tetrauoride are vaporized and removed While continuously adding water to said slurry to maintain a substantially constant boiling composition boiling higher than about 250 F., and Withdrawing a phosphoric acid-silica slurry from the boiling operation having a P/F weight ratio in excess of 100.

12. The method of claim 11 wherein said added silica is a diatomaceous earth.

References Cited in the file of this patent UNITED STATES PATENTS 1,858,203 Fiske et al. May 10, 1932 2,132,349 Booth Oct. 4, 1938 2,165,100 Hettrick July 4, 1939 2,354,177 Kawecki July 18, 1944 2,526,776 Smith et aL Oct. 24, 1950 2,726,928 Hollingsworth Dec. 13, 1955 

1. THE METHOD OF REMOVING FLUORINE FROM WET PROCESS PHOSPHORIC ACID CONTAINING FLUORINE IN UNDESIRABLE AMOUNTS WHICH COMPRISES HEATING SAID ACID AT A CONCENTRATION HAVING A BOILING POINT HIGHER THAN ABOUT 250* F. TO A BOILING TEMPERATURE IN THE PRESENCE OF IN EXCESS OF ABOUT 0.1% BY WEIGHT OF SILICA HAVING A SURFACE AREA IN EXCESS OF 25 SQUARE METERS PER GRAM, BOILINT THE RESULTANT ACID SLURRY WHEREBY WATER VAPOR AND SILICON TETRAFLUORIDE ARE VAPORIZED AND REMOVED WHILE CONTINUOUSLY ADDING WATER TO MAINTAIN A SUBSTANTIALLY CONSTANT BOILING COMPOSITION BOILING HIGHER THAN ABOUT 250*F., WITHDRAWING A PHOSPHORIC ACID-SILICA SLURRY FROM THE BOILING OPERATION HAVING A P/F WEIGHT RATIO IN EXCESS OF 100, AND REMOVING SILICA FROM THE DEFLUORINATED ACID. 